1. Field of the Invention
The present invention relates to high alumina content ceramic compositions, sintered ceramics formed therefrom and to processes for manufacturing the same.
2. Description of the Prior Art
One recent advance in electronics packaging to meet microminiaturization demands in the art has been the development of multilayer ceramic sandwiches; as are described in "Laminated Ceramics," Proc. Electron. Comp. Conf. (Washington, D.C., 1967), page 17; "Ceramics for Packaging," Solid State Technology, 14, 1971, page 40; "A Fabrication Technique for Multilayer Ceramic Modules," Solid State Technology, May 1972, page 35; and "Metal-Ceramic Constraints for Multilayer Electronic Packages," Proceedings of the IEEE, 59, 1455 (1971).
Multilayer ceramic sandwiches find particular usage in computer circuitry, and accordingly stringent requirements are posed thereon both from a processing viewpoint and from the viewpoint of final device requirements.
For instance, multilayer ceramic sandwiches usually comprise a plurality of ceramic substrates in combination with internal metallization, the combination having been sintered at elevated temperatures, especially in the range of 1,400.degree.-1,700.degree. C. Metallurgy having a melting point higher than the sintering temperature is generally used, typically refractory metals such as molybdenum or tungsten. However, the refractory metals require the use of reducing ambients during sintering and critical process control since the partial pressure of oxygen must be maintained low enough during sintering to prevent oxidation of the metal and yet must not be so low that silica present in the ceramic is extensively reduced. Further, the use of refractory metals has also necessitated stringent controls on the physical properties of the ceramic used so that mechanical failure of the ceramic does not occur, for instance, due to stresses arising from thermal expansion mismatches between the metallurgy and the ceramic.
Further, under such a reducing ambient, prior art ceramic materials often blister or mottle, unless the particle size of the starting ceramic is critically controlled.
U.S. Pat. No. 3,020,619 Koch discloses forsterite ceramic compositions having as the major crystalline phase 2MgO.sup.. SiO.sub.2. The material is made synthetically from, for example, Montana talc, fused MgO, potash feldspar, Kentucky Special ball clay and precipitated BaCO.sub.3.
U.S. Pat. No. 3,480,452 Fleischner, et al. disclose a method of making void free crystalline-glass ceramic materials from two frits, one being a thermally crystallizable MgO-Al.sub.2 O.sub.3 -SiO.sub.2 glass and the other phase being 10-30% of a bonding frit of the formula: MgO-CaO-BaO-Al.sub.2 O-SiO.sub.2, the resultant body containing a cordierite crystalline phase.
U.S. Pat. No. 3,489,627 Botden, et al. disclose a bonding composition and a method for bonding using the same, the bonding composition being substantially composed of CaO, BaO and/or SiO.sub.2, to which MgO, SrO and/or Al.sub.2 O.sub.3 may be added, in which case the proportion of Al.sub.2 O.sub.3 is at most 75% by weight.
U.S. Pat. No. 3,615,763 Flock relates to sintered ceramic articles useful as electrical insulators consisting essentially of a reaction product which, calculated as oxides, is approximately 94-96.5 wt. % Al.sub.2 O.sub.3 and a mixture of SiO.sub.2, CaO and MgO.
U.S. Pat. No. 3,631,131 Kopko discloses a method for reconstituting unfired, cast alumina scrap wherein Al.sub.2 O.sub.3 can be blended with SiO.sub.2, MgCO.sub.3, CaCO.sub.3 and a binder such as polyvinyl butyral to form a firing charge.
U.S. Pat. No. 3,698,923 Stetson, et al. disclose a fired alumina ceramic material which can comprise 96% Al.sub.2 O.sub.3, 2% CaSiO.sub.3 and 2% MgSiO.sub.3, fired at 1500.degree. C.
Miller et al. in Ceramic Bulletin, Vol. 48, No. 8 (1969), page 786, disclose BaO-Al.sub.2 O.sub.3 -SiO.sub.2 glasses, and discuss the same in detail.
Floyd, Journal of the American Ceramic Society, Vol. 47, No. 11, November 21, 1964, discusses the effect of secondary crystalline phases on dielectric losses in high-alumina bodies, and concludes that the presence of one of the three feldspars, BaO.sup.. Al.sub.2 O.sub.3.sup.. 2SiO.sub.2, CaO.sup.. Al.sub.2 O.sub.3.sup.. 2SiO.sub.2 or SrO.sup.. Al.sub.2 O.sub.3.sup.. 2SiO.sub.2 cause high dielectric losses when present in high-alumina bodies.
Goodyear, et al. in the Ceramic Bulletin, Vol. 45, No. 8 (1966), pages 706, et seq., present an investigation of the CaO-Al.sub.2 O.sub.3 -SiO.sub.2 system. This reference relates to the formation of "vitrified" ceramics such as cordierite or anorthite. Special glass frits are added as batch constituents, along with other raw materials, to yield desired overall compositions of the exact compositions of the phases such as anorthite or cordierite. The composition of anorthite, and the final composition of the ceramic in Goodyear, et al., is CaO.sup.. Al.sub.2 O.sub.3.sup.. 2SiO.sub.2. The present application relates to the formation of high alumina ceramics with properties completely different from the properties of vitrified ceramics such as anorthite.
Caldwell and Gdula have suggested the addition of baria, magnesia and silica to alumina as fluxing agents. However, the prior art believed that a silica:alkaline earth ratio of 1.5:1 was needed to render such bodies impervious to water (low porosity). Such compositions fall within the "two liquid region" for the baria-magnesia-silica system and illustrate a tendency to phase separation. The best of such compositions had the following analysis: Al.sub.2 O.sub.3 91%; SiO.sub.2 5.60%; MgO 1.90%; BaO 1.50%; F 1.06%. Caldwell and Gdula indicate that glass prepared from components of the system BaO-MgO-SiO.sub.2 can be used to fabricate ceramic bodies which densify to negligible water absorption if the silica:alkaline earth ratio in the glass is 1.5:1, the ceramic is made from 4 .mu. Al.sub.2 O.sub.3 and is fired in air. In reducing ambients and with different particle size aluminas the behavior of the glass fluxing aids are much different and compositions different from those indicated were found to be most useful in accordance with the present invention. In addition, the best composition in Caldwell and Gdula was one containing fluoride. This type of glass is not useful in hydrogen sintering ambients.
The prior art has also suggested a glass sintering aid for alumina containing four components: Al.sub.2 O.sub.3, CaO, MgO and SiO.sub.2. Such ceramics are extremely susceptible to blistering during sintering and occasional mottling on the surface of the sintered ceramic also occurs. To obtain good glass sintering aid qualities, it is further necessary to have appreciable percentages of TiO.sub.2, Fe.sub.2 O.sub.3, Na.sub.2 O and K.sub.2 O.